fix(gnovm): allow type-switch with sole case nil: (preprocess single-case branch must tag-type-only when ct is nil)

[ltzmaxwell]

• Before: a type-switch whose sole case was case nil: (e.g. switch xx := x.(type) { case nil: ... }) panicked at preprocess with name xx not declared because the single-case shortcut registered the switch var with anyValue(ct) where ct == nil; after: the sole-nil form is accepted and xx is bound to the tag's static type, matching Go.
• preprocess1's *TypeSwitchStmt branch in gnovm/pkg/gnolang/preprocess.go now gates the len(n.Cases) == 1 define-with-case-type shortcut on ct != nil, so a nil ct falls through to the same tag-type define used for multi-case switches.
• New filetest gnovm/tests/files/typeswitch1.gno mirrors gocorpus's typeswitch1 (sole case nil: alongside mixed multi-type cases) and exercises both the xx := x.(type) and bare xx.(type) shapes.

[Gno2D2]

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